linux-kernel - Re: [PATCH] sched/fair: improve spreading of utilization

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Message-ID: <jhj4kuspgse.mognet@arm.com>
Date:   Fri, 13 Mar 2020 12:42:41 +0000
From:   Valentin Schneider <valentin.schneider@....com>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     Valentin Schneider <valentin.schneider@....com>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] sched/fair: improve spreading of utilization


On Fri, Mar 13 2020, Valentin Schneider wrote:
> On Fri, Mar 13 2020, Vincent Guittot wrote:
>>> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>>> > index 3c8a379c357e..97a0307312d9 100644
>>> > --- a/kernel/sched/fair.c
>>> > +++ b/kernel/sched/fair.c
>>> > @@ -9025,6 +9025,14 @@ static struct rq *find_busiest_queue(struct lb_env *env,
>>> >               case migrate_util:
>>> >                       util = cpu_util(cpu_of(rq));
>>> >
>>> > +                     /*
>>> > +                      * Don't try to pull utilization from a CPU with one
>>> > +                      * running task. Whatever its utilization, we will fail
>>> > +                      * detach the task.
>>> > +                      */
>>> > +                     if (nr_running <= 1)
>>> > +                             continue;
>>> > +
>>>
>>> Doesn't this break misfit? If the busiest group is group_misfit_task, it
>>> is totally valid for the runqueues to have a single running task -
>>> that's the CPU-bound task we want to upmigrate.
>>
>>  group_misfit_task has its dedicated migrate_misfit case
>>
>
> Doh, yes, sorry. I think my rambling on ASYM_PACKING / reduced capacity
> migration is still relevant, though.
>

And with more coffee that's another Doh, ASYM_PACKING would end up as
migrate_task. So this only affects the reduced capacity migration, which
might be hard to notice in benchmarks.